We couldn’t have chosen more appropriate weather (hot!) to mark the release of our report looking at changing summer heat in the Midwest. Millennium Park in Chicago was just beginning to get really hot and Crown Fountain crowded as we discussed the summer heat trends that Chicago has experienced over the past 60 or so years. A true “DT” day, indeed. (You’ll have to read the report to find out what that means. No spoilers here). The report finds that, on average, summer weather is changing in ways that increase the risk of heat-related health impacts in large Midwestern cities, including Chicago, Minneapolis, Detroit, St. Louis, and Cincinnati.

Hot summer weather greeted the UCS report release in Chicago.

There are many ways to measure heat, including maximum temperature and heat index. We chose to look at what are called air masses. Think of the large umbrella of air over a city that is described by its temperature, dew point (a measure of humidity), wind, and cloudiness. We were ultimately concerned with how changing summer weather impacts human health. Air masses do this nicely as they capture multiple weather variables that the body responds to in one fell swoop.

Increasing trends in dangerous air masses

We focused on two types of air masses that have historically been associated with negative impacts to human health. The first is the very dry and hot air mass. The other is hot and humid.

We looked at the most extreme subset of the hot and humid air masses. We also took a look at a class of air masses that are dry and cool. These are the nice, summer days that provide relief from the heat. It would stand to reason that if more summer days are coming in the form of stifling heat then there would be less cool days.

The short version of the story is that most of the cities we looked at are now, on average, seeing more hot and humid and hot and dry days per summer than they did five or six decades ago. And to no real surprise these trends are accompanied by an overall decrease in cool and dry summer days.

We also found rather strong trends in increasing overnight temperatures in the dangerous air masses. This has health implications since people often rely on cooler nighttime temperatures to provide relief from oppressive daytime heat. It appears that for the dangerous air masses this relief may be decreasing as time goes on.

Finally, the number of three or more consecutive day events of these hot air masses occurring each summer has also increased. The number of these “heat waves” may at first seem modest as we’re seeing increases of up to four per summer over the record. However, the impacts of any one event can be substantial. A one-off day of hot weather can be manageable, but string together many days in a row and the situation can become dangerous.

Climate change increasing risk of heat-related health impacts

One thing our study did not do is to directly determine if the trends we are seeing in the Midwest are due to human activities. However, we looked at smaller, partner cities and found similar trends to the larger cities. This implies that the heat trends in the larger cities are not being driven by urban heat island effects alone. The picture that emerged is consistent with a general climate trend. Looking at 60 years of data also rules out the case of trends arising from short-term events, such as a strong La Nina.

Heat presents a real threat to human health. It has been responsible for more deaths than all other natural disasters combined over the period 1979-2003. The trends we found aren’t based on models, but on real weather data over the past six decades. This is what folks in the Midwest have actually experienced. This also appears to be a risk that is not going away. Many studies have projected increases in future heat waves and temperatures in general, including for the Midwest. All of our study cities are currently engaged, at some level, with adaptation planning as we discuss in the report, but will it be enough for a hotter future?

About the author:
Todd Sanford is a climate scientist with expertise in the atmospheric chemistry and physics of the climate system. His current work involves the public health impacts of climate change. He holds a PhD in physical chemistry from the University of Colorado. See Todd's full bio.

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